At least 15 soldiers and an official from Nigeria’s disaster agency were killed in an ambush by suspected Boko Haram militants in the northeast of the country, security sources said on Thursday, weeks after 20 troops went missing in an attack.

The ambush occurred late on Wednesday in the northern Damasak area of Borno, the state worst hit by the jihadist group which has killed more than 30,000 people since 2009, when it launched an insurgency to create an Islamic caliphate.

It highlights the challenge to secure the northeast months ahead of a February election President Muhammadu Buhari plans to contest in which security looks set to be a campaign issue.

In July a fourth commander in 14 months was named to lead the fight against the militants after a number of embarrassing defeats.

Two soldiers and a staff member from the National Emergency Management Agency (NEMA) said that 15 troops and a NEMA official who was with them were killed in the latest incident.

“We lost 15 troops. Our men were carrying out digging of trenches at Damasak yesterday when the Boko Haram terrorists opened fire on them,” said a soldier who did not want to be named. The NEMA worker, who also did not want to be named, said 18 were killed.

NEMA also released a statement in which it confirmed the death of an official in the attack.

“The Director General of the National Emergency Management Agency, Engr. Mustapha Maihaja and the management staff have been thrown into mourning following a death of a staff who was killed by Boko Haram yesterday in Damasak, Borno state,” it said.

The deaths come after 20 Nigerian soldiers went missing in mid-July following a clash with militants in the Bama area of Borno. Military sources say the troops are feared dead.

Boko Haram had held territory around the size of Belgium in northeast Nigeria for several months until being pushed off much of that land in early 2015 by Nigeria’s army and soldiers from neighbouring countries.

Nigeria’s government has said since December 2015 that the insurgents have been “technically defeated”. Yet attacks continue in the northeast, while another group, an Islamic State ally that split from Boko Haram in 2016, also holds some territory.